1. Field of the Invention
The present invention relates to an image forming apparatus such as an electrophotographic copying machine or a printer.
2. Description of the Related Art
The following method is conventionally known as a method (hereinafter, referred as an “image control method”) of adjusting image density characteristics of an image forming apparatus such as a copying machine or a printer.
The image forming apparatus forms a tone pattern including multiple patches on an image bearing member such as a photosensitive drum upon completion of warm-up operation after startup. The image forming apparatus then reads a density of the formed tone pattern, and based on the read density value, changes an operation of a circuit such as a gamma correcting circuit for determining image forming conditions, to thereby stabilize the quality of an image to be formed (for example, refer to Japanese Patent Application Laid-Open No. 2000-238341).
When fluctuations in environmental conditions change the tone characteristics of the image forming apparatus, a tone pattern is formed again on the image bearing member, read, and fed back again to the circuit such as a gamma correcting circuit for determining image forming conditions. As a result, image quality may be stabilized according to the fluctuations in environmental conditions.
When the image forming apparatus is used over a long period of time, where a read density of the tone pattern on the image bearing member and a density of an actually printed-out image may not match each other. Thus, there is known a method of forming a tone pattern on a recording material, and correcting image forming conditions based on the density value of the tone pattern (for example, refer to Japanese Patent Application Laid-Open No. H07-131650).
There is known a method of correcting a gamma look-up table (γLUT) or creating a γLUT correction table based on density information of one image pattern, and adding correction information to the gamma correcting circuit so as to compensate for the shortfall in correction information. In this regard, calibration is known to be performed for correcting a change in output characteristics caused by an individual difference of the apparatus such as a printer or a change in output environment (for example, refer to Japanese Patent Application Laid-Open No. 2003-202711).
The above-mentioned conventional technologies have the following problems. Specifically, a tone pattern having a patch of a fixed density level is conventionally used for creating a tone correction table. On the other hand, a great change in tone reproduction may not always occur at the same place, depending on variance on engine characteristics between the apparatus, environmental changes or materials.
However, the density level of formed patches is fixed, and hence it is difficult to sufficiently correct a density level of a great tone change. The number of patches to be formed may be increased to preferentially correct the density level of which change in tone reproduction is great. However, the increase in the number of patches may result in an extended period of down-time.